As a conventional transmission circuit, for example, there is a transmission circuit that generates a transmission signal by using a quadrature modulation scheme. Since the transmission circuit using the quadrature modulation scheme is widely known, the description thereof will be omitted. Further, as a conventional transmission circuit which operates more efficiently than a quadrature modulation circuit, for example, there is a known modulation circuit that uses an envelope elimination and restoration (EER) modulation scheme or a polar modulation scheme. According to such modulation schemes, an input signal is separated into a phase component signal and an amplitude component signal. First, the phase component signal is multiplied by an oscillation signal generated by an oscillator to generate a phase modulation signal with a constant amplitude. Next, by using a saturation amplifier, the amplitude component signal is amplified and synthesized with the phase modulation signal to generate a transmission signal.
The EER modulation scheme features that the saturation amplifier is used to synthesize the amplitude component with the phase modulation signal. The EER modulation scheme further features that it can generate a transmission signal with a high power efficiency because the saturation amplifier is operated in a saturation region.
However, when an output level of the transmission signal is low, in the EER modulation scheme, power efficiency may deteriorate due to the deviation of the amplifier from the saturation region, or power efficiency may deteriorate due to an increase in a ratio of the power consumption of an amplitude modulation section to the entire power consumption. By this means, conventionally, a transmission circuit has been suggested which improves the power efficiency by producing a linear operation of an amplifier using the EER modulation scheme when the output is high and using the quadrature modulation scheme when the output is low (e.g., Patent Literature 1 (FIG. 6)). The operation of the transmission circuit disclosed in Patent Literature 1 will be described below (e.g., Patent Literature 1 (FIG. 6)).
An interface section receives an in-phase component signal (I signal) and a quadrature component signal (Q signal), which are quadrature scheme signals, from a baseband section. An Rθ conversion section of the interface section switches between the quadrature modulation scheme and the EER modulation scheme based on an auto gain control (AGC) control signal from the baseband section. In the quadrature modulation scheme, the Rθ conversion section performs through-output on the I signal and the Q signal without processing the signals, and in the EER modulation scheme, the Rθ conversion section converts (Rθ conversion process) the I signal and the Q signal into an amplitude component signal and a phase component signal, respectively. The Rθ conversion process is performed by extracting phase information and detecting an envelope by a limiter.
A digital-to-analog converter (DAC) receives as input the I signal when using the quadrature modulation scheme, whereas the DAC receives as input the phase component signal when using the EER scheme. Further, the DAC receives as input the Q signal when using the quadrature modulation scheme, whereas the DAC receives as input the amplitude component signal when using the EER modulation scheme.
A switch couples the output of the DAC to a Q component baseband filter of a radio frequency-integrated circuit (RF-IC) in the quadrature modulation scheme, and the switch couples the output of the DAC to an amplitude modulation signal in the EER modulation scheme.
A switch couples the sum of the I signal and the Q signal to an AGC amplifier in the quadrature modulation scheme, and the switch couples the phase component signal to the AGC amplifier in the EER modulation scheme. The switch couples the output of the AGC amplifier to an output buffer and outputs the resultant to a front-end section without bypassing a power amplifier in the quadrature modulation scheme, and the switch couples the output of the AGC amplifier to the power amplifier to amplify the output of the AGC amplifier in the EER modulation scheme.
In the quadrature modulation scheme, the input signal is converted into the I signal and the Q signal. The oscillation signal generated by the oscillator is separated into two signals by a phase shifter. One signal thereof is output to a mixer without phase shifting, and then is multiplied by the I signal by the mixer. The other signal thereof is output to the mixer after phase shifting, and then is multiplied by the Q signal by the mixer. The signals multiplied by the I signal and the Q signal are subsequently synthesized with each other by an adder, so that the synthesized signal becomes a modulation wave in accordance with the quadrature modulation scheme.
In accordance with the EER modulation scheme, the input signal is converted into the amplitude component signal and the phase component signal. The oscillation signal generated by the oscillator is first multiplied by the phase component signal to generate the phase modulation signal. Then, the power amplifier performs amplitude synthesis on the phase modulation signal and the amplitude component signal, so that a modulation wave is formed in accordance with the EER modulation scheme.
That is, the transmission circuit switches the modulation scheme, so that the quadrature modulation is performed when the power level of the signal is lower than a given value and the EER modulation is performed when the power level of the signal is higher than the given value. In this way, the conventional transmission circuit realizes a reduction in power consumption by using the quadrature modulation scheme and the EER modulation scheme in a combination manner.